Add a Position struct

[metalgearsloth]

For most APIs we can just use Vector2 still (e.g. circle position, Box2, etc) but transform should be using a position struct that can be swapped out between 32-64bit precision (as to how we deliver it, separate engine builds or smth IDK but I think the performance difference is immaterial enough to want double-precision).

See https://github.com/erincatto/box2d/blob/56edae79f2949d86142b03450d5d60f63bcf5a6f/docs/large_worlds.md